/sys$common/syshlp/helplib.hlb BACKUP Command, Qualifiers, /TAPE_EXPIRATION  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /TAPE_EXPIRATION=dd-mmm-yyyy

    Output Save-Set Qualifier

    Writes a file expiration date other than the current date to the
    file header label of the save set.

    Compaq recommends that you specify an expiration date whenever
    you create a BACKUP save set on magnetic tape using /REWIND.
    Daily BACKUP tapes should expire in seven days, weekly BACKUP
    tapes should expire in one month, and monthly BACKUP tapes should
    expire in one year. Specify the date in absolute time format,
    according to the rules described the OpenVMS User's Manual. If
    you do not specify an expiration date, today's date is written to
    the volume header record when you perform a save operation using
    /REWIND.